Switchable K–O bonds unveiling anomalous ferroelastic transitions in a polar hybrid crystal

Hybrid ferroelastic crystals have garnered considerable interest due to their promising potential as mechanical switches and sensors. The anomalous ferroelastic phase transitions, in which ferroelasticity occurs in the high-temperature phase rather than the low-temperature phase, are of particular interest, but they are sporadically-documented and none of them is involved in breaking of the chemical bonds. Herein, a hydroxyl-containing cation, i.e., Me₃NOH⁺, is employed to construct a three-dimensional hybrid crystal (Me₃NOH)₂KBiCl₆ (1). This crystal undergoes distinct two-step structural phase transitions with space-group changes of Pna2₁–P112₁–P6₃mc, belonging to an anomalous temperature-reversed mm2F2 ferroelastic transition and a normal 6mmF2 ferroelastic transition, respectively. The anomalous ferroelastic transition is entirely driven by switchable K–O coordination bonds involving breaking and reformation. Notably, the dynamic behavior of Me₃NOH⁺ cations along with the distortion of inorganic framework enables the manifestation of unusual “high-low-medium” second-harmonic generation-switching behaviors. This study presents the enormous benefits of switchable coordination bonds for inducing anomalous ferroelastic phase transitions, offering valuable insight for the exploration of new multifunctional ferroelastic materials.